This invention concerns punch holders, and more particularly punch holders for use in turret punch presses where a number of holders are carried by rotary turrets for selective engagement of the press ram with one of the punch holders. A stripping arrangement is necessary to be incorporated allowing the punch to be withdrawn after the workpiece has been punched without lifting of the workpiece as the punch, tightly fit to the punched hole, is retracted. Lifting of the workpiece degrades the quality of the punched hole, as well as other problems well known in the art.
The necessary stripping force levels, typically created by springs, can be considerable, particularly for thicker workpieces, and lifting of the workpiece often occurs notwithstanding the inclusion of a stripper arrangement.
Prior stripper arrangements have not allowed completely adequate stripper spring forces, particularly for holders for turret punch presses which are of relatively compact design. The springs cannot be overly stressed or their service life is excessively shortened. A typical multiple spring stripper uses a series of studs, each mounting an individual spring, this arrangement being relatively bulky. See U.S. Pat. No. 3,496,818 for an example of this type. Belleville washer sets have also been used, but while relatively compact, have not resulted in completely adequate stripper force levels.
A stripper plate is typically included in the prior art stripper arrangements, the stripper plate directly bearing against the workpiece surface, under the pressure of the stripper springs used to generate the stripping force.
The stripper plate has an opening closely fit to the punch for the most effective stripping.
The stripper plate sometimes needs to be of a relatively soft material such a brass or urethane plastic to avoid marring of the surface of the workpiece. The stripper plate thus needs to be changed with a change in workpiece material, and may be constructed of a variety of materials such as brass, plastic, steel, with an opening closely fit to the particular punch.